ISOLATION AND IDENTIFICATION OF A PLASMATOCYTE-SPREADING PEPTIDE FROMTHE HEMOLYMPH OF THE LEPIDOPTERAN INSECT PSEUDOPLUSIA-INCLUDENS

Insect blood cells (hemocytes) play an essential role in defense again st parasites and other pathogenic organisms that infect insects. A key class of hemocytes involved in insect cellular immunity is plasmatocy tes. Here we describe the isolation and identification of a peptide fr om the moth Pseudoplusia includens that mediates the spreading of plas matocytes to foreign surfaces. This peptide, designated plasmatocyte-s preading peptide (PSP1), contains 23 amino acid residues in the follow ing sequence: H-ENFNGGCLAGYMRTADGRCKPTF-OH. In vitro assays using the synthetic peptide at concentrations greater than or equal to 2 nM indu ced plasmatocytes from P. includens to spread an the surface of cultur e dishes. Injection of this peptide into P. includens larvae caused a transient depletion of plasmatocytes from circulation, Labeling studie s indicated that this peptide induced 75% of plasmatocytes that were d ouble-labeled by the monoclonal antibodies 49G3A3 and 43E9A8 to spread , whereas plasma induced significantly more plasmatocytes to spread, T his suggests that only a certain subpopulation of plasmatocytes respon ds to the peptide and that other peptidyl factors mediate plasmatocyte adhesion responses.